The invention relates to a system and method for controlling an internal combustion engine coupled to an emission control device.
In direct injection spark ignition engines, the engine operates at or near wide-open throttle during stratified air-fuel ratio operation in which the combustion chambers contain stratified layers of different air-fuel ratio mixtures. Strata closest to the spark plug contain a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. The engine may also operate in a homogeneous mode of operation with a homogeneous mixture of air and fuel generated in the combustion chamber by early injection of fuel into the combustion chamber during its intake stroke. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry.
Direct injection engines are also coupled to emission control devices known as three-way catalytic converters optimized to reduce CO, HC, and NOx. When operating at air-fuel ratio mixtures lean of stoichiometry, a three way catalyst optimized for NOx storage, known as a NOx trap or catalyst, is typically coupled downstream of the first three-way catalytic converter. The NOx trap typically stores NOx when the engine operates lean and release NOx to be reduced when the engine operates rich or near stoichiometry.
One method of determining when to operate end lean operation and regenerate a NOx trap by operating the engine rich or near stoichiometry uses an estimate of NOx discharged from the NOx trap. In this approach, when an estimated amount of NOx discharged from the NOx trap is greater than a predetermined value, the NOx trap is regenerated.
The inventors herein have recognized a disadvantage with the above approach. In particular, during transient vehicle conditions, there is more error in the estimated NOx discharged. Therefore, to accommodate all operating conditions, the threshold is set low. Then, when operating at steady conditions, the low threshold causes sub-optimal fuel economy performance than could otherwise be achieved.
An object of the invention claimed herein is to provide a method for controlling an internal combustion engine that can optimize fuel efficiency across all vehicle operating conditions.
The above object is achieved, and disadvantages of prior approaches overcome, by a method for controlling an internal combustion engine of a vehicle having a tail pipe, the engine coupled to an emission control device, the method comprising: generating an indication of vehicle activity based on an operating condition; determining a set-point amount of emissions from the tail pipe per distance of the vehicle, with said set-point based on said indication; and adjusting an engine operating parameter based on said set-point.
By adjusting the tailpipe threshold based on vehicle activity, it is possible to optimize performance during all conditions. advantage of the above aspect of the present invention is improved fuel economy.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.